Eighteen Month Development in Twin Lowland Gorillas at Zoo Atlanta

Twin (1.1) lowland gorillas (Gorilla gorilla gorilla) were born to a group-living mother on Nov. 1, 2005 at Zoo Atlanta. The gorilla group had been formed a year earlier; there were 2 additional surviving infants born into the group in the months following the birth of the twins. Observations of the infants were made by a trained observer for 30-minute sessions 2 times per week. This report focuses on the 18 month development of the twins, describing changes in a variety of behavioral variables, including a decline in time spent in contact from 30 min per observation to 5 min per observation as the infants aged, and an increase in distance spent from one another. Infants began leaving and approaching the mothers in the third month, reaching as high as 9 leaves per observation in the later months. The infants first moved beyond 1m in the sixth month and beyond 5m in the tenth month. Infants became more outward directed as self manipulation declined and object examination and social play increased with increasing age. The mother retrieved the infants regularly, reaching a peak of 6 retrievals per hour in the fifth month, and began following them as the infants moved away. A number of behaviors were compared with other gorilla infants raised using Wilcoxon Signed Ranks Test [a=0.05, two-tailed] and there were few differences found between the twins and the comparison infants. The twins showed significantly lower levels of object examination, sitting and leaving the mother than the comparison infants.